The need for an efficient and accurate user positioning in wireless networks grows day by day. The position estimation of wireless callers to emergency numbers, as the “Enhanced-911” (E-911) mandatory in the U.S. by October 2001 and the E-112 under consideration by the European Community, is only one of the possible applications beside vehicle fleet management, intelligent transport systems (ITS's) and location-based billing.
To guarantee positioning in the whole coverage area, achieve different service requirements and handle the variety of location information, combinations of different location technologies have been considered and suitable signaling protocols proposed. Standardization of Mobile Stations (MS's) positioning in GSM networks started in 1995 within ETSI SMG (European Telecommunications Standards Institute Special Group Mobile) 1 after European operators asked for commercial services based on MS positioning and separately within T1P1 in the U.S., after the FCC (Federal Communications Commission) established the requirements for the E-911 service.
In 1997 the T1P1.5 subcommittee of T1P1 (T1P1 is a subgroup of T1, a committee of TIA, the North American Telecommunications Industry Association) was given the responsibility to lead the standardization activities. At the conclusion of the works, T1P1.5 proposed four positioning alternatives: three using measurements performed within the cellular network; namely, the Timing Advance (TA), Uplink Time Of Arrival (TOA) and Enhanced Observed Time Difference (E-OTD) procedures and one based on the Global Positioning System (GPS) technology, the Network-Assisted GPS procedure. All four procedures were included by ETSI in the GSM specifications Release '98 and Release '99 (see ETSI, “Digital Cellular Telecommunications System (Phase 2+); Location Services (LCS), (Functional Description)—Stage 2 (GSM 03.71 version 7.3.0 Release 1998)”—doc. ETSI TS 101 724, February 2000).
The Uplink TOA procedure is able to locate legacy handsets (i.e., handsets with standard software) but requires installation of new network elements called LMUs (Location Measurement Units) at each BTS site. The Enhanced OTD procedure requires installation of LMUs every 2-5 BTS's and software modification in the terminals. Also the Network-Assisted GPS procedure requires installation of reference GPS receivers besides the integration of a GPS receiver into the handset.
All procedures, including the Timing Advance, require the introduction of a new network element responsible for the location calculation called SMLC (Serving Mobile Location Center).
The principle of the Timing Advance is illustrated in FIG. 1. The Timing Advance (TA) describes the back-and-forth propagation time between the mobile station and the serving base station. It is used to avoid overlapping slots in communications. The distance d between the serving base station and the mobile station can be estimated using the TA:d≅TA×(cTb/2)where Tb=3.69 μs is the bit period, c=3×108 m/s is the speed of radio waves, and TA is an integer ranging from 0 to 63.
The Timing Advance procedure is the simplest procedure among the standardized ones. It requires only the addition of location calculation functionalities (i.e., the SMLC) in the network side and, since it uses information already available in nowadays networks, it is able to locate legacy handsets and it does not need the installation of additional measurement units.
The Timing Advance procedure can be applied also as back-up procedures for other standardized solutions. In some cases, in fact, the number of Base Transceiver Stations (BTS's) in case of Uplink TOA/Enhanced OTD or satellites, in case of Network-Assisted GPS, that can be used for positioning purposes is low or the quality of the measurement is so poor that none of such technologies can be used. The estimations obtained by using the Timing Advance technique can also be used as initial guess for the algorithms used to implement one of the standardized solutions (e.g. the E-OTD), in order to improve accuracy or speed of convergence of those algorithms.